The healing process after a small cut could lead to skin tumors, scientists find. (But don’t panic! This happens if someone is already predisposed to having that cancer, and the cut would just increase the chances.)

Stem cells in hair follicles can have cancer-causing mutations (when errors in DNA accumulate). And after injury to the skin, these stem cells could lead to a common form of skin cancer called basal cell carcinoma.

These stem cells in hair follicles differentiate and divide to replace hair when it falls out, and in order for the skin to heal itself, these follicular stem cells can also help regenerate tissues. But these processes have been poorly understood.

So Sunny Wong and Jeremy Reiter from University of California, San Francisco created mice that can produce follicular stem cells that have the gene that's implicated in basal cell carcinoma. These genetically engineered mice also come equipped with an on/off switch for this cancer-causing, mutated gene (also called oncogene).

Switching on this oncogene – known as Smoothened – wasn’t enough to trigger cancer in the mouse.

But, when the oncogene was turned on around the same time as the mouse skin was damaged, the follicular stem cells with the activated oncogene migrated to the sites of injury, and the mouse developed tumors at the wound sites.

Wounds ranging from small paper-cut-like incisions to pencil-eraser-sized holes punched in the back all caused these stem cells to form tumors nearby, Science News reports, but plucking single hairs did not.

Wong and Reiter found that the stem cells with mutated genes stayed near the follicles, in the lower layers of skin – until the mice were wounded. Once the animals had been cut, the cells migrated to the upper layers of skin to fix the damage, but while there, they disrupted a biochemical signaling pathway that has been linked to basal cell carcinoma development – and thus seeded cancer growth.

"It's surprising that activating oncogenes in a stem-cell population doesn't cause tumors," Reiter says. "This gives us a glimpse of a way in which our bodies protect us." A variety of cancers are associated with wounds like burns and battlefield injuries.

The researchers also found that the stem cells trigger tumors even when the wound occurred several weeks after the oncogene was switched on.

This makes real the scary threat that dormant stem cells may accumulate cancer-causing mutations until some tipping point – like a paper cut – wakes up the cells and prompts the formation of tumors.

In fact, Reiter admits that when he first found that injuries could trigger cancer, he "started to get worried about shaving".

Janet Fang has written for Nature, Discover and the Point Reyes Light. She is currently a lab technician at Lamont-Doherty Earth Observatory. She holds degrees from the University of California, Berkeley and Columbia University. She is based in New York.
Follow her on Twitter.